1. Field of the Invention
This invention relates to a process for stably producing a solid-liquid metal mixture in which non-dendritic primary solid particles are dispersed into the remaining liquid matrix (hereinafter referred to as a semi-solidified metal composition).
2. Disclosure of the Related Art
The term "semi-solidified metal composition" used herein means that molten metal (generally molten alloy) is vigorously agitated while cooling convert dendrites produced in the remaining liquid matrix into such a state having a spheroidal or granular shape that dendritic branches substantially eliminate or reduce (which is called as non-dendritic primary solid particles) and then disperse these primary solid particles into the liquid matrix.
As disclosed in, for example, U.S. Pat. No. 3,902,544, there is a process for the production of the semi-solidified metal composition, wherein molten metal is vigorously agitated in a cylindrical cooling agitation vessel through high rotation of an agitator while cooling to convert dendrites produced in the remaining liquid matrix into non-dendritic primary solid particles in which dendritic branches eliminate or reduce into a spheroidal or granular shape, and then these non-dendritic primary solid particles are dispersed into the liquid matrix to form a slurry of semi-solidified metal composition, which is discharged from a nozzle disposed as the bottom of the cooling agitation vessel continuously or at once every one charge of molten metal.
In the conventional process, it is known to conduct mechanical agitation using the above agitator, electromagnetic agitation electromagnetically agitating molten metal in the cooling agitation vessel and the like.
In general, the fluidity of the resulting semi-solidified metal composition is dependent upon fraction solid, increasing rate of fraction solid (represented by a ratio of solid phase metal to total volume of semi-solidified metal slurry) per unit time at solid-liquid coexistent state (hereinafter referred to as solidification rate) and average value of rate change per unit distance of the liquid matrix influenced by the agitating speed (hereinafter referred to as shear rate). In the conventional technique, therefore, it is frequently difficult to stably produce the semi-solidified metal composition because even when the fraction solid is same, the flowing of the semi-solidified metal composition is stopped in the cooling agitation vessel to cause problems such as impossibility of discharging the composition, the clogging of the discharge port with the composition and the like.
The fluidity of the semi-solidified metal composition is generally degraded as the fraction solid becomes high. When the fraction solid is not less than a certain value, usually not less than about 0.65, there are caused problems that the semi-solidified metal composition can not be discharged from the production apparatus or transferred into subsequent multi-stage production apparatus for the semi-solidified metal composition, casting device, holding device or working device to cause the stop of the flowing of the semi-solidified metal composition in the cooling agitation vessel, the impossibility of discharging the semi-solidified metal composition due to the clogging, solidification or the like.
Even when the fraction solid is not more than 65%, the fluidity becomes poor as the solidification rate is large or the shear rate is small. In other words, it is necessary that a relation of fluidity (viscosity) exerting on not only the fraction solid of the semi-solidified metal composition and solidification rate but also the shear rate is clarified in order to conduct the stable production of the semi-solidified metal composition and the stable discharge and transfer of the semi-solidified metal composition into subsequent multi-stage production apparatus, casting device, holding device and working device, whereby the agitation at a shear rate met with the fraction solid of the semi-solidified metal composition and the cooling rate or the cooling at a cooling rate met with the shear rate is conducted to properly control the fluidity.
On the other hand, when the amount of solid metal in the semi-solidified metal composition (called as fraction solid) exceeds a certain limit value due to external factors such as temperature of molten metal poured for the continuous production, discharge rate of the semi-solidified metal composition, cooling rate and the like, the viscosity of the semi-solidified metal composition rapidly increases to exhibit no fluid behavior and it is impossible to discharge the semi-solidified metal composition from the production apparatus.
In order to detect such a change of the viscosity, there has hitherto been proposed a method wherein the temperature of the semi-solidified metal composition discharged from the production apparatus is measured to estimate the fraction solid discharged, whereby the fraction solid causing the impossible discharge is controlled. In this method, there is a time lag between the cooling of molten metal and the discharge of the semi-solidified metal composition, so that it is very difficult to susceptibly control the fraction solid and hence it is difficult to stably produce the semi-solidified metal composition for a long time.